1. Field of the Invention
The present invention relates to a system for controlling head movement in a disc storage unit in which a multiphase stepping motor is used for displacing a head and in which in response to the reception of a series of pulses for designating the number of stepping displacements, the head is controlled to be displaced in accordance with the designated number of stepping displacements.
2. Description of the Prior Art
When accessing a disc, typically a fixed disc, a seeking operation is carried out to displace the head onto a track along which the desired data is recorded by selecting the track out of hundreds of tracks defined over one or both surfaces of the disc. As the means for moving the head, stepping motors have been widely used. As the name implies, the stepping motor displaces the head stepwise. The stepping motor has a plurality of exciting phases (generally, two exciting phases). The principle of operations of the stepping motor is similar to conventional motors in general. The clockwise or counterclockwise of a rotor of the motor is performed by switching vectors which are defined by a plurality of phase currents (exciting currents) flowing through coils of the motor corresponding to a plurality of exciting phases. In the access operation, the computer designates a logic address of desired data which in turn is converted into a physical address including the track number of the track along which the desired data is recorded, so that a difference in number between the track number a desired track upon which the head is to be positioned and that of a track upon which the head is positioned at present is computed. Both of a direction signal, which represents a sign of the difference, and a series of step pulses, which include pulses, the number of which is equal to the numerical value of the difference, are supplied to the disc storage unit.
In the disc storage unit, after receiving the series of step pulses, the vectors are sequentially switched in accordance with the switching times designated by step pulses and in the direction designated by the direction signal, with sequentially switching the phase currents by the signal D, so that the stepping motor is driven to displace the head stepwise in the designated direction.
If, however, the pulse rate of the series of step pulses is considerably low (for instance 10 milliseconds per pulse), it takes as long as five microsecond to receive the pulses which designate the number of steps, in the case of recently developed disc storage units in which more than 500 tracks are defined over one surface of the disc. Accordingly, it is difficult to shorten the access time.